The Axis advance halted in 1942 when Japan lost the critical
Battle of Midway, and Germany and Italy were
defeated in North Africa
and then, decisively, at Stalingrad
in the Soviet Union. In 1943, with a series of German defeats on the Eastern Front, the
Allied invasions of Sicily
and Italy
which brought about Italian surrender, and Allied victories in the Pacific, the Axis lost the initiative and was forced into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained all of its territorial losses and invaded Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in
SouthCentral China
and Burma, while the Allies crippled the
Japanese Navy
and captured key Western Pacific islands.

World War II changed the political alignment and social structure of the world. The
United Nations
(UN) was established to foster international co-operation and prevent future conflicts. The victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—became the
permanent members
of the United Nations Security Council.[7]
The Soviet Union and the United States emerged as rival superpowers, setting the stage for the
Cold War, which lasted for the next 46 years. Meanwhile, the influence of European great powers waned, while the
decolonisation of Africa
and Asia
began. Most countries whose industries had been damaged moved towards economic recovery. Political integration, especially
in Europe, emerged as an effort to end pre-war enmities and to create a common identity.[8]

The German Empire was dissolved in the
German Revolution of 1918–1919, and a democratic government, later known as the
Weimar Republic, was created. The interwar period saw strife between supporters of the new republic and hardline opponents on both the
right
and left. Italy, as an Entente ally, had made some post-war territorial gains; however, Italian nationalists were angered that the
promises made
by Britain and France to secure Italian entrance into the war were not fulfilled in the peace settlement. From 1922 to 1925, the Fascist
movement led by Benito Mussolini
seized power in Italy with a nationalist, totalitarian, and
class collaborationist
agenda that abolished representative democracy, repressed socialist, left-wing and liberal forces, and pursued an aggressive expansionist foreign policy aimed at making Italy a world power, promising the creation of a "New Roman Empire".[22]

When civil war broke out in Spain, Hitler and Mussolini lent military support to the
Nationalist rebels, led by General
Francisco Franco. The Soviet Union supported the existing government, the
Spanish Republic. Over 30,000 foreign volunteers, known as the
International Brigades, also fought against the Nationalists. Both Germany and the USSR used this
proxy war
as an opportunity to test in combat their most advanced weapons and tactics. The Nationalists won the civil war in April 1939; Franco, now dictator, remained officially neutral during World War II but generally favoured the Axis.[36]
His greatest collaboration with Germany was the sending of volunteers
to fight on the Eastern Front.[37]

Soviet–Japanese border conflicts

In the mid-to-late 1930s, Japanese forces in
Manchukuo
had sporadic border clashes with the Soviet Union and the Mongolian People's Republic. The Japanese doctrine of
Hokushin-ron, which emphasised Japan's expansion northward, was favoured by the Imperial Army during this time. With the Japanese defeat at
Khalkin Gol
in 1939, the ongoing Second Sino-Japanese War[49]
and ally Nazi Germany pursuing neutrality with the Soviets, this policy would prove difficult to maintain. Japan and the Soviet Union eventually signed a Neutrality Pact
in April 1941, and Japan adopted the doctrine of Nanshin-ron, promoted by the Navy, which took its focus southward, eventually leading to its war with the United States and the Western Allies.[50][51]

The situation reached a general crisis in late August as German troops continued to mobilise against the Polish border. In August 23, when tripartite negotiations about a military alliance between France, Britain and USSR stalled,[58]
the Soviet Union signed a non-aggression pact
with Germany.[59]
This pact had a secret protocol that defined German and Soviet "spheres of influence" (western Poland
and Lithuania
for Germany; eastern Poland, Finland,
Estonia, Latvia and
Bessarabia
for the USSR), and raised the question of continuing Polish independence.[60]
The pact neutralized the possibility of Soviet opposition to a campaign against Poland and assured that Germany would not have to face the prospect of a two-front war, as it had in World War I. Immediately after that, Hitler ordered the attack to proceed on 26 August, but upon hearing that Britain had concluded a formal mutual assistance pact with Poland, and that Italy would maintain neutrality, he decided to delay it.[61]

In response to British requests for direct negotiations to avoid war, Germany made demands on Poland, which only served as a pretext to worsen relations.[62]
On 29 August, Hitler demanded that a Polish plenipotentiary
immediately travel to Berlin to negotiate the handover of Danzig, and to allow a
plebiscite
in the Polish Corridor
in which the German minority would vote on secession.[62]
The Poles refused to comply with the German demands, and on the night of 30–31 August in a violent meeting with the British ambassador Neville Henderson, Ribbentrop declared that Germany considered its claims rejected.[63]

Germany
annexed
the western and occupied
the central part of Poland, and the USSR annexed
its eastern part; small shares of Polish territory were transferred to Lithuania
and Slovakia. On 6 October, Hitler made a public peace overture to Britain and France, but said that the future of Poland was to be determined exclusively by Germany and the Soviet Union. The proposal was rejected,[63]
and Hitler ordered an immediate offensive against France,[71]
which would be postponed until the spring of 1940 due to bad weather. [72][73][74]

Finnish machine gun nest aimed at Soviet
Red Army
positions during the Winter War, February 1940

The Soviet Union forced the
Baltic countries—Estonia, Latvia and Lithuania, the states that were in the Soviet "sphere of influence" under the Molotov-Ribbentrop pact—to sign
"mutual assistance pacts"
that stipulated stationing Soviet troops in these countries. Soon after, significant Soviet military contingents were moved there.[75][76][77]
Finland refused to sign a similar pact and rejected ceding part of its territory to the USSR. The Soviet Union invaded Finland in November 1939,[78]
and the USSR was expelled from the League of Nations.[79]
Despite overwhelming numerical superiority, Soviet military success was modest, and the Finno-Soviet war
ended in March 1940 with minimal Finnish concessions.[80]

In November 1939, the United States was taking measures to assist China and the Western Allies, and amended the
Neutrality Act
to allow "cash and carry"
purchases by the Allies.[103]
In 1940, following the German capture of Paris, the size of the United States Navy
was significantly increased. In September the United States further agreed to a
trade of American destroyers for British bases.[104]
Still, a large majority of the American public continued to oppose any direct military intervention in the conflict well into 1941.[105]
In December 1940 Roosevelt accused Hitler of planning world conquest and ruled out any negotiations as useless, calling for the US to become an "arsenal of democracy" and promoting
Lend-Lease
programmes of aid to support the British war effort.[99]
The US started strategic planning to prepare for a full-scale offensive against Germany.[106]

At the end of September 1940, the
Tripartite Pact
formally united Japan, Italy and Germany as the Axis Powers. The Tripartite Pact stipulated that any country, with the exception of the Soviet Union, which attacked any Axis Power would be forced to go to war against all three.[107]
The Axis expanded in November 1940 when Hungary, Slovakia and Romania
joined.[108]Romania
and Hungary
would make major contributions to the Axis war against the USSR, in Romania's case partially to recapture territory ceded to the USSR.[109]

In early June 1940 the Italian
Regia aeronauticaattacked and beseiged Malta, a British possession. In late summer through early autumn Italy
conquered British Somaliland
and made an incursion into British-held Egypt. In October
Italy attacked Greece, but the attack was repulsed with heavy Italian casualties; the campaign ended within days with minor territorial changes.[110]
Germany started preparation for an invasion of the Balkans to assist Italy, to prevent the British from gaining a foothold there, which would be a potential threat for Romanian oil fields, and to strike against the British dominance of the Mediterranean.[111]

With the situation in Europe and Asia relatively stable, Germany, Japan, and the Soviet Union made preparations. With the Soviets wary of mounting tensions with Germany and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in
Southeast Asia, the two powers signed the
Soviet–Japanese Neutrality Pact
in April 1941.[120]
By contrast, the Germans were steadily making preparations for an attack on the Soviet Union, massing forces on the Soviet border.[121]

Hitler believed that Britain's refusal to end the war was based on the hope that the United States and the Soviet Union would enter the war against Germany sooner or later.[122]
He therefore decided to try to strengthen Germany's relations with the Soviets, or failing that to attack and eliminate them as a factor. In November 1940, negotiations took place
to determine if the Soviet Union would join the Tripartite Pact. The Soviets showed some interest, but asked for concessions from Finland, Bulgaria, Turkey, and Japan that Germany considered unacceptable. On 18 December 1940, Hitler issued the directive to prepare for an invasion of the Soviet Union.

German soldiers during the invasion of the Soviet Union by the
Axis powers, 1941

By October Axis
operational objectives
in Ukraine and the Baltic region were achieved, with only the sieges of Leningrad[136]
and Sevastopol
continuing.[137]
A major offensive against Moscow
was renewed; after two months of fierce battles in increasingly harsh weather the German army almost reached the outer suburbs of Moscow, where the exhausted troops[138]
were forced to suspend their offensive.[139]
Large territorial gains were made by Axis forces, but their campaign had failed to achieve its main objectives: two key cities remained in Soviet hands, the Soviet capability to resist
was not broken, and the Soviet Union retained a considerable part of its military potential. The blitzkriegphase
of the war in Europe had ended.[140]
By early December, freshly mobilised reserves[141]
allowed the Soviets to achieve numerical parity with Axis troops.[142]
This, as well as intelligence data
which established that a minimal number of Soviet troops in the East would be sufficient to deter any attack by the Japanese Kwantung Army,[143]
allowed the Soviets to begin a massive counter-offensive
that started on 5 December all along the front and pushed German troops 100–250 kilometres (62–155 mi) west.[144]

War breaks out in the Pacific (1941)

In 1939, the United States had renounced its trade treaty with Japan; and, beginning with an
aviation gasoline
ban in July 1940, Japan became subject to increasing economic pressure.[99]
During this time, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by late September.[145]
Despite several offensives
by both sides, the war between China and Japan was stalemated by 1940. To increase pressure on China by blocking supply routes, and to better position Japanese forces in the event of a war with the Western powers, Japan invaded and occupied northern Indochina.[146]
Afterwards, the United States embargoed
iron, steel and mechanical parts against Japan.[147]
Other sanctions soon followed.

German successes in Europe encouraged Japan to increase pressure on European governments in
Southeast Asia. The Dutch government agreed to provide Japan some oil supplies from the
Dutch East Indies, but negotiations for additional access to their resources ended in failure in June 1941.[152]
In July 1941 Japan sent troops to southern Indochina, thus threatening British and Dutch possessions in the Far East. The United States, United Kingdom and other Western governments reacted to this move with a freeze on Japanese assets and a total oil embargo.[153][154]
At the same time, Japan was planning an invasion of the Soviet Far East, intending to capitalise off the German invasion in the west, but abandoned the operation after the sanctions.[155]

Since early 1941 the United States and Japan had been engaged in negotiations in an attempt to improve their strained relations and end the war in China. During these negotiations Japan advanced a number of proposals which were dismissed by the Americans as inadequate.[156]
At the same time the US, Britain, and the Netherlands engaged in secret discussions for the joint defence of their territories, in the event of a Japanese attack against any of them.[157]
Roosevelt reinforced the Philippines (an American protectorate scheduled for independence in 1946)
and warned Japan that the US would react to Japanese attacks against any "neighboring countries".[157]

Frustrated at the lack of progress and feeling the pinch of the American-British-Dutch sanctions, Japan prepared for war. On 20 November a new government under Hideki Tojo presented an interim proposal as its final offer. It called for the end of American aid to China and for the supply of oil and other resources to Japan. In exchange Japan promised not to launch any attacks in Southeast Asia and to withdraw its forces from southern Indochina.[156]
The American counter-proposal of 26 November required that Japan evacuate all of China without conditions and conclude non-aggression pacts with all Pacific powers.[158]
That meant Japan was essentially forced to choose between abandoning its ambitions in China, or seizing the natural resources it needed in the Dutch East Indies by force;[159]
the Japanese military did not consider the former an option, and many officers considered the oil embargo an unspoken declaration of war.[160]

These attacks led the
United States,
United Kingdom, China, Australia and several other states to formally declare war on Japan, whereas the Soviet Union, being heavily involved in large-scale hostilities with European Axis countries, maintained its neutrality agreement with Japan.[164]
Germany, followed by the other Axis states, declared war on the United States[165]
in solidarity with Japan, citing as justification the American attacks on German war vessels that had been ordered by Roosevelt.[123][166]

During 1942, Allied officials debated on the appropriate
grand strategy
to pursue. All agreed that defeating Germany
was the primary objective. The Americans favoured a straightforward, large-scale attack
on Germany through France. The Soviets were also demanding a second front. The British, on the other hand, argued that military operations should target peripheral areas to wear out German strength, leading to increasing demoralisation, and bolster resistance forces. Germany itself would be subject to a heavy bombing campaign. An offensive against Germany would then be launched primarily by Allied armour without using large-scale armies.[169]
Eventually, the British persuaded the Americans that a landing in France was infeasible in 1942 and they should instead focus on driving the Axis out of North Africa.[170]

At the
Casablanca Conference
in early 1943, the Allies reiterated the statements issued in the 1942 Declaration by the United Nations, and demanded the unconditional surrender
of their enemies. The British and Americans agreed to continue to press the initiative in the Mediterranean by invading Sicily to fully secure the Mediterranean supply routes.[171]
Although the British argued for further operations in the Balkans to bring Turkey into the war, in May 1943, the Americans extracted a British commitment to limit Allied operations in the Mediterranean to an invasion of the Italian mainland and to invade France in 1944.[172]

In early May 1942, Japan initiated operations to
capture Port Moresby
by amphibious assault
and thus sever communications and supply lines between the United States and Australia. The planned invasion was thwarted when an Allied task force, centred on two American fleet carriers, fought Japanese naval forces to a draw in the Battle of the Coral Sea.[179]
Japan's next plan, motivated by the earlier Doolittle Raid, was to seize
Midway Atoll
and lure American carriers into battle to be eliminated; as a diversion, Japan would also send forces to occupy the Aleutian Islands
in Alaska.[180]
In mid-May, Japan started the Zhejiang-Jiangxi Campaign
in China, with the goal of inflicting retribution on the Chinese who aided the surviving American airmen in the Doolittle Raid by destroying air bases and fighting against the Chinese 23rd and 32nd Army Groups.[181][182]
In early June, Japan put its operations into action but the Americans, having broken Japanese naval codes
in late May, were fully aware of plans and order of battle, and used this knowledge to achieve a decisive victory at Midway
over the Imperial Japanese Navy.[183]

Eastern Front (1942–43)

Despite considerable losses, in early 1942 Germany and its allies stopped a major Soviet offensive in central and southern Russia, keeping most territorial gains they had achieved during the previous year.[190]
In May the Germans defeated Soviet offensives in the Kerch Peninsula
and at Kharkov,[191]
and then launched their main summer offensive
against southern Russia in June 1942, to seize the oil fields of the Caucasus
and occupy Kubansteppe, while maintaining positions on the northern and central areas of the front. The Germans split
Army Group South
into two groups: Army Group A
advanced to the lower Don River
and struck south-east to the Caucasus, while Army Group B
headed towards the Volga River. The Soviets decided to make their stand at Stalingrad on the Volga.[192]

In June 1943 the British and Americans began
a strategic bombing campaign
against Germany with a goal to disrupt the war economy, reduce morale, and "de-house" the civilian population.[211]
The firebombing of Hamburg
was among the first attacks in this campaign, it lead to significant casualties and inflicted considerable losses on infrastructure of this important industrial center.[212]

On 12 July 1943, the Soviets launched their own
counter-offensives, thereby dispelling any chance of German victory or even stalemate in the east. The Soviet victory at Kursk marked the end of German superiority,[219]
giving the Soviet Union the initiative on the Eastern Front.[220][221]
The Germans tried to stabilise their eastern front along the hastily fortified Panther–Wotan line, but the Soviets broke through it at
Smolensk
and by the Lower Dnieper Offensives.[222]

By the start of July 1944, Commonwealth forces in Southeast Asia had repelled the Japanese sieges in Assam, pushing the Japanese back to the
Chindwin River[258]
while the Chinese captured Myitkyina. In September 1944, Chinese force captured the Mount Song
to reopen the Burma Road.[259]
In China, the Japanese had more successes, having finally captured Changsha in mid-June and the city of Hengyang
by early August.[260]
Soon after, they invaded the province of Guangxi, winning major engagements against Chinese forces at Guilin and Liuzhou
by the end of November[261]
and successfully linking up their forces in China and Indochina by mid-December.[262]

Axis collapse, Allied victory (1944–45)

On 16 December 1944, Germany made a last attempt on the Western Front by using most of its remaining reserves to launch
a massive counter-offensive in the Ardennes
and along the French–German border
to split the Western Allies, encircle large portions of Western Allied troops and capture their primary supply port at Antwerp
to prompt a political settlement.[264]
By January, the offensive had been repulsed with no strategic objectives fulfilled.[264]
In Italy, the Western Allies remained stalemated at the German defensive line. In mid-January 1945, the Soviets and Poles attacked in Poland, pushing from the Vistula to the Oder
river in Germany, and overran East Prussia.[265]
On 4 February, Soviet, British and US leaders met for the Yalta Conference. They agreed on the occupation of post-war Germany, and on when the Soviet Union would join the war against Japan.[266]

In May 1945, Australian troops
landed in Borneo, over-running the oilfields there. British, American, and Chinese forces defeated the Japanese in northern
Burma
in March, and the British pushed on to reach Rangoon
by 3 May.[275]
Chinese forces started to counterattack in Battle of West Hunan
that occurred between 6 April and 7 June 1945. American naval and amphibious forces also moved towards Japan, taking Iwo Jima
by March, and Okinawa
by the end of June.[276]
At the same time, American submarines cut off
Japanese imports, drastically reducing Japan's ability to supply its overseas forces.[277]

The Allies established occupation administrations in
Austria
and Germany. The former became a neutral state, non-aligned with any political bloc. The latter was divided into western and eastern occupation zones controlled by the Western Allies and the USSR, accordingly. A
denazification
programme in Germany led to the prosecution of Nazi war criminals
and the removal of ex-Nazis
from power, although this policy moved towards amnesty and re-integration of ex-Nazis into West German society.[284]

Post-war division of the world was formalised by two international military alliances, the United States-led
NATO
and the Soviet-led Warsaw Pact;[304]
the long period of political tensions and military competition between them, the Cold War, would be accompanied by an unprecedented
arms race
and proxy wars.[305]

The global economy suffered heavily from the war, although participating nations were affected differently. The US emerged much richer than any other nation; it had a
baby boom
and by 1950 its gross domestic product per person was much higher than that of any of the other powers and it dominated the world economy.[311]
The UK and US pursued a policy of industrial disarmament in Western Germany
in the years 1945–1948.[312]
Because of international trade interdependencies this led to European economic stagnation and delayed European recovery for several years.[313][314]

The Soviet Union, despite enormous human and material losses, also experienced rapid increase in production in the immediate post-war era.[323]
Japan experienced incredibly rapid
economic growth, becoming one of the most powerful economies in the world by the 1980s.[324]
China returned to its pre-war industrial production by 1952.[325]

The Soviet Union lost around 27 million people during the war,[329]
including 8.7 million military and 19 million civilian deaths.[330]
A quarter of the people in the Soviet Union were wounded or killed.[331]
Germany sustained 5.3 million military losses, mostly on the Eastern Front and during the final battles in Germany.[332]

In Asia and the Pacific, between 3 million and more than 10 million civilians, mostly Chinese (estimated at 7.5 million[341]), were killed by the Japanese occupation forces.[342]
The most infamous Japanese atrocity was the Nanking Massacre, in which fifty to three hundred thousand Chinese civilians were raped and murdered.[343]
Mitsuyoshi Himeta reported that 2.7 million casualties occurred during the Sankō Sakusen. General
Yasuji Okamura
implemented the policy in Heipei and Shantung.[344]

The German government led by
Adolf Hitler
and the Nazi Party
was responsible for the Holocaust
(killing of approximately 6 million Jews), as well as for killing of 2.7 million ethnic Poles,[353]
and 4 million others who were deemed "unworthy of life" (including the
disabled
and mentally ill,
Soviet prisoners of war,
homosexuals,
Freemasons,
Jehovah's Witnesses, and
Romani) as part of a programme of deliberate extermination.
Soviet POWs
were kept in especially unbearable condition, and, although their extermination was not an official goal, 3.6 million of Soviet POWs out of 5.7 died in Nazi camps during the war.[354][355]
In addition to concentration camps,
death camps
were created in Nazi Germany to exterminate people at an industrial scale. Nazi Germany extensively used forced labourers. About 12 million
Europeans
from German occupied countries were used as slave work force in German agriculture and war economy.[356]
Soviet Gulag
became de facto
a system of deadly camps during 1942–43, when wartime privation and hunger caused numerous deaths of inmates,[357]
including foreign citizens of Poland and other countries
occupied in 1939–40 by the USSR, as well as of the Axis POWs.[358]
By the end of the war, most Soviet POWs liberated from Nazi camps and many repatriated civilians were detained in special filtration camps where they were subjected to NKVD
check, and significant part of them was sent to Gulag as real or perceived Nazi collaborators.[359]

Prisoner identity photograph taken by the German
SS
of a Polish girl deported
to Auschwitz. Approximately 230,000 children were held prisoner, and used in forced labor and medical experiments.

At least five million Chinese civilians from northern China and Manchukuo were enslaved between 1935 and 1941 by the
East Asia Development Board, or
Kōain, for work in mines and war industries. After 1942, the number reached 10 million.[363]
In Java, between 4 and 10 million
rōmusha
(Japanese: "manual labourers"), were forced to work by the Japanese military. About 270,000 of these Javanese labourers were sent to other Japanese-held areas in South East Asia, and only 52,000 were repatriated to Java.[364]

In Europe, occupation came under two forms. In Western, Northern, and Central Europe (France, Norway, Denmark, the Low Countries, and the
annexed portions of Czechoslovakia) Germany established economic policies through which it collected roughly 69.5 billion reichmarks (27.8 billion US dollars) by the end of the war, this figure does not include the
sizeable plunder
of industrial products, military equipment, raw materials and other goods.[365]
Thus, the income from occupied nations was over 40 per cent of the income Germany collected from taxation, a figure which increased to nearly 40 per cent of total German income as the war went on.[366]

In the East, the intended gains of
Lebensraum
were never attained as fluctuating front-lines and Soviet scorched earth
policies denied resources to the German invaders.[367]
Unlike in the West, the Nazi racial policy
encouraged extreme brutality against what it considered to be the "inferior people" of Slavic descent; most German advances were thus followed by
mass executions.[368]
Although resistance groups
formed in most occupied territories, they did not significantly hamper German operations in either the East[369]
or the West[370]
until late 1943.

In Asia, Japan termed nations under its occupation as being part of the
Greater East Asia Co-Prosperity Sphere, essentially a Japanese
hegemony
which it claimed was for purposes of liberating colonised peoples.[371]
Although Japanese forces were originally welcomed as liberators from European domination in some territories, their excessive brutality turned local public opinion against them within weeks.[372]
During Japan's initial conquest it captured 4,000,000 barrels (640,000 m3) of oil (~5.5×105
tonnes) left behind by retreating Allied forces, and by 1943 was able to get production in the Dutch East Indies up to 50 million barrels (~6.8×10^6 t), 76 per cent of its 1940 output rate.[372]

Home fronts and production

In Europe, before the outbreak of the war, the Allies had significant advantages in both population and economics. In 1938, the Western Allies (United Kingdom, France, Poland and British Dominions) had a 30 per cent larger population and a 30 per cent higher gross domestic product than the European Axis powers (Germany and Italy); if colonies are included, it then gives the Allies more than a 5:1 advantage in population and nearly 2:1 advantage in GDP.[373]
In Asia at the same time, China had roughly six times the population of Japan, but only an 89 per cent higher GDP; this is reduced to three times the population and only a 38 per cent higher GDP if Japanese colonies are included.[373]

The United States provided about two-thirds of all the ordnance used by the Allies in terms of warships, transports, warplanes, artillery, tanks, trucks, and ammunition.[374]
Though the Allies' economic and population advantages were largely mitigated during the initial rapid blitzkrieg attacks of Germany and Japan, they became the decisive factor by 1942, after the United States and Soviet Union joined the Allies, as the war largely settled into one of attrition.[375]
While the Allies' ability to out-produce the Axis is often attributed to the Allies having more access to natural resources, other factors, such as Germany and Japan's reluctance to employ women in the labour force,[376]
Allied strategic bombing,[377]
and Germany's late shift to a war economy[378]
contributed significantly. Additionally, neither Germany nor Japan planned to fight a protracted war, and were not equipped to do so.[379]
To improve their production, Germany and Japan used millions of slave labourers;[380]Germany used
about 12 million people, mostly from Eastern Europe,[356]
while Japan used
more than 18 million people in Far East Asia.[363][364]

Advances in technology and warfare

Aircraft were used for
reconnaissance, as
fighters,
bombers, and
ground-support, and each role was advanced considerably. Innovation included
airlift
(the capability to quickly move limited high-priority supplies, equipment, and personnel);[381]
and of strategic bombing
(the bombing of enemy industrial and population centres to destroy the enemy's ability to wage war).[382]Anti-aircraft weaponry
also advanced, including defences such as radar
and surface-to-air artillery. The use of the jet aircraft
was pioneered and, though late introduction meant it had little impact, it led to jets becoming standard in air forces worldwide.[383]

In the Atlantic,
escort carriers
proved to be a vital part of Allied convoys, increasing the effective protection radius and helping to close the Mid-Atlantic gap.[387]
Carriers were also more economical than battleships because of the relatively low cost of aircraft[388]
and their not requiring to be as heavily armoured.[389]
Submarines, which had proved to be an effective weapon during the First World War,[390]
were anticipated by all sides to be important in the second. The British focused development on anti-submarineweaponry
and tactics, such as sonar
and convoys, while Germany focused on improving its offensive capability, with designs such as the Type VII submarine
and wolfpack
tactics.[391]
Gradually, improving Allied technologies such as the Leigh light,
hedgehog,
squid, and
homing torpedoes
proved victorious.

Land warfare
changed from the static front lines of World War I to increased mobility and combined arms. The
tank, which had been used predominantly for infantry support in the First World War, had evolved into the primary weapon.[392]
In the late 1930s, tank design was considerably more advanced than it had been during World War I,[393]
and advances continued throughout the war
with increases in speed, armour and firepower.

At the start of the war, most commanders thought enemy tanks should be met by tanks with superior specifications.[394]
This idea was challenged by the poor performance of the relatively light early tank guns against armour, and German doctrine of avoiding tank-versus-tank combat. This, along with Germany's use of combined arms, were among the key elements of their highly successful blitzkrieg tactics across Poland and France.[392]
Many means of destroying tanks, including
indirect artillery,
anti-tank guns
(both towed and self-propelled),
mines, short-ranged infantry antitank weapons, and other tanks were used.[394]
Even with large-scale mechanisation, infantry remained the backbone of all forces,[395]
and throughout the war, most infantry were equipped similarly to World War I.